This invention relates generally to passive thermal management of batteries and portions thereof, and more particularly to the use of a heat pipe with a pressure relief device to reduce the risk of thermal propagation of heat generated within the battery during a thermal event.
Lithium-ion and related batteries, collectively known as a rechargeable energy storage system (RESS), are being used in automotive applications as a way to supplement, in the case of hybrid electric vehicles (HEVs), or supplant, in the case of purely electric vehicles (EVs), conventional internal combustion engines (ICEs). The ability to passively store energy from stationary and portable sources, as well as from recaptured kinetic energy provided by the vehicle and its components, makes batteries ideal to serve as part of a propulsion system for cars, trucks, buses, motorcycles and related vehicular platforms. In the present context, a cell is a single electrochemical unit, whereas a battery is made up of one or more cells joined in series, parallel or both, depending on desired output voltage and capacity.
Temperature is one of the most significant factors impacting both the performance and life of a battery. Environment temperatures (such as those encountered during protracted periods of inactivity in cold or hot environments, or due to extended periods of operation and concomitant heat generation on hot days) or abuse conditions (such as the rapid charge/discharge, or internal/external shorts caused by the physical deformation, penetration, or manufacturing defects of the cells) can negatively impact the ability of the battery to operate correctly, and in severe cases can destroy the battery entirely. Side effects of prolonged exposure to high temperature may include premature aging and accelerated power and energy fade, both of which are undesirable.
Excess heat can be provided by an external source or by the internal failure of a battery cell caused by physical, thermal, or operational abuse conditions outside the recommended tolerances, or by manufacturer defects. The temperature at which an exothermic reaction occurs is known as the onset temperature. The heat required to maintain an exothermic reaction is the heat of reaction. Provision of a heat source that exceeds the onset temperature and maintains the heat of reaction is known as a thermal event. Such thermal events, if left uncontrolled, could potentially lead to a more accelerated heat generation condition called thermal runaway, a condition where (once initiated) the cooling mechanism is incapable of returning one or more of the battery components to a safe operating temperature. In the present context, a thermal runaway is a function of the self heating rate of the exothermic reaction and the temperature, and the time of the reaction is a function of the rate of degradation and the mass of active components taking part in the reaction. Of particular concern is the possibility for excess heating of, and related damage to, a battery cell, pack or related member being used as a source of propulsive power. Conventional heat transfer methods such as forced air and liquid cooling, whether as a primary or backup system, may prove to be effective at avoiding such excessive heat exposure during such a thermal event, but do so by significant additions to overall vehicular system weight, cost, complexity or parasitic power requirements.
Heat pipes may be used to perform some heat transfer solutions mentioned above; however, the manufacture of a heat pipe may also fraught with difficulties. In one form, the pipe is first built and then filled with a suitable heat transfer fluid (typically de-ionized water), after which the internal part of the pipe and the material container within is fluidly isolated from the ambient environment. Such an arrangement is potentially subject to failure due to excess internal pressure, contamination, or chemical degradation of the heat transfer fluid if exposed to too much heat during subsequent manufacture (such as welding or otherwise affixing the heat pipe to a cooling fin, plate, heat sink or other structure). One way around this challenge is the pursuit of a post-process fill of the heat transfer fluid into the heat pipe; however, such an approach is prone to contamination of either the heat transfer medium or assembly, either of which could result in the failure of the operation of the heat pipe.